This invention relates to the field of catalysts, especially supported catalysts and in particular to plurimetallic catalysts, their production and uses.
The major refining and petrochemical processes use heterogeneous catalysis to a large extent: the catalyst comprises a carrier and a metal in various forms (as oxide, sulfide, reduced metal). In some cases, the carrier may also take part in the reaction.
The requirement of more and more stable and selective catalysts has given incentive to much research, one essential result of which has been the formation of improved catalysts comprising several metals: a new generation of plurimetallic catalysts was thus created. This evolution has resulted in very substantial improvements in stability and selectivity.
The conventional preparation techniques by exchange from an aqueous solution of the one or more compounds which have to be fixed on the carrier lead to a macroscopically homogeneous distribution of the different metals over the whole volume of the carrier particle. Sometimes, successive impregnations of each metal are required. Moreover, this technique of impregnation with an aqueous solution may have the major disadvantage of modifying the surface structure of the carrier: acid attack, for example. All these features seem to indicate that the conventional procedure for obtaining a homogeneous phase containing two or more metals is far from being perfectly controlled.
An object of the present invention is to provide a method for the preparation of improved plurimetallic catalysts as well as the catalysts produced thereby.
This method consists of contacting a hydrocarbon compound of a metal selected from the group consisting of germanium, tin and lead with a catalyst containing one or more group VIII metals, selected from the platinum, palladium, nickel and cobalt group, deposited on a carrier. The proportion of element selected from the germanium, tin and lead group is from 0.1 to 10% by weight and more particularly from 0.2 to 6%. The proportion by weight of the one or more group VIII metals deposited on the carrier depends on the metal concerned; this proportion ranges from 0.1 to 5% and more advantageously from 0.2 to 2% for the platinum, iridium, rhodium, ruthenium and palladium group; it is from 0.1 to 10% and particularly from 1 to 5% for the nickel and cobalt group. The catalyst may also contain molybdenum.
The carrier may be selected from the group consisting of silica, different alumina types, silica-aluminas, coal and, preferably, from the various alumina types.
The catalyst may be prepared by different procedures of carrier impregnation and the invention is not limited to a particular one. The preparation preferably comprises two steps: a first one of fixing the one or more group VIII metals and then, after optional thermal activation, the fixation of one or more metals selected from the germanium, tin and lead group.
The fixation of the group VIII metal may be achieved by different methods. The impregnation consists, for example, of contacting the preformed carrier with an aqueous solution of a compound of the selected metal, the volume of the solution being in excess with respect to the retention volume of the carrier or equal to said volume. After having maintained the solution in contact with the carrier for several hours, the impregnated carrier is filtered, washed with distilled water, then dried under scavenging with air or an inert gas. The so-impregnated carrier may also be subjected to calcination in air at a temperature higher than 120.degree. C. but, in everyy case, lower than 550.degree. C.
The metal selected from the tin, germanium and lead group may be introduced onto the carrier already preimpregnated with the group VIII metal and optionally calcined or calcined and reduced, as a hydrocarbon solution of a hydrocarbyl-tin, hydrocarbyl-germanium and/or hydrocarbyl-lead compound. The contact between the preimpregnated carrier and the solution is maintained for several days. The carrier, thus impregnated with one or more group VIII metals and one or more metals from the tin, germanium and lead group, may undergo several washings with a hydrocarbon, for example with the hydrocarbon selected as solvent for the impregnation with the compound of metal(s) selected from the tin, germanium and lead group. After the washing step, the catalyst is dried, for example in a stream of air or inert gas (nitrogen, argon, helium) at a temperature from 80.degree. to 120.degree. C. After drying, the catalyst may optionally be calcined in air between 110.degree. and 550.degree. C. and preferably between 110.degree. and 450.degree. C.
Before use, the catalyst may be reduced, for example in hydrogen atmosphere, between 200.degree. and 600.degree. C. and preferably between 300.degree. and 500.degree. C., this reduction being optionally performed just after the drying or the calcination or subsequently by the user.
The group VIII metal may be supplied from such compounds as chlorides, nitrates, acetylacetonates or organic acid salts soluble in the impregnation solvent. Organometallic compounds of group VIII metals may be used as solution in a hydrocarbon, e.g. a saturated paraffinic hydrocarbon whose hydrocarbon chain contains 6 to 12 carbon atoms, a naphthenic hydrocarbon containing 6 to 12 carbon atoms or an aromatic hydrocarbon containing 6 to 11 carbon atoms; the preference will be given to acetylacetonates of group VIII metals.
The element selected from the tin, germanium and lead group may be introduced as an alkyl- or aryl-compound of said metals, for example as tetrabutyl-tin, tetramethyl-tin, tetrapropyl-germanium, tetraethyl-lead, diphenyl-tin, diphenyl-germanium or tetraphenyl-lead in hydrocarbon solution. These compounds preferably conform with the formula Me(R)n wherein Me=Sn, Ge or Pb, R=hydrocarbyl of 1-12 carbon atoms, identical or different, and n is an integer, preferably 2 or 4.
As already mentioned, various types of carriers can be used. A particularly suitable carrier is that having specific characteristics such as a specific surface, determined by the B.E.T. method, from 10 to 500 square meters per gram, preferably from 50 to 500 square meters per gram, and a total pore volume from 20 to 130 cc per 100 grams of carrier and preferably from 40 to 100 cc per 100 grams of carrier.
After deposition of the metals on the carrier, the catalyst is advantageously subjected to an activation treatment in hydrogen at high temperature, for example 200.degree.-500.degree. C., so as to obtain an active metal phase.
This treatment in hydrogen consists, for example, in a slow temperature rise under hydrogen steam up to a maximum reduction temperature ranging from 200.degree. to 500.degree. C. and, preferably, from 260.degree. to 400.degree. C., followed with the maintenance of said temperature for 1 to 10 hours.
These catalysts have particularly interesting properties for the selective hydrogenations of acetylenic and diolefinic compounds (particularly hydrocarbons), in the optional presence of sulfur and nitrogen compounds. They can also be used as catalysts for hydrodesulfurizing saturated or unsaturated hydrocarbon charges; their use results in a substantial decrease of the hydrogenation rate of unsaturated molecules (olefins or aromatics), this being of particular advantage when treating a light gasoline.